1. Field of the Invention
The present invention relates to an optical scanning unit for forming an image on an image-forming surface by scanning a laser beam, which is applicable to a laser printer, laser facsimile, digital copy machine, and the like.
2. Description of the Related Art
A laser printer operating on a semiconductor laser device, has a photosensitive drum. Around the photosensitive drum, arranged are a charging device, an exposing device, a developing device, a transferring device, and the like, and a recording sheet feeding device and a fixing device are arranged respectively on the upstream and downstream sides of the photosensitive drum.
The exposing device includes an optical scanning unit consisting of a semiconductor laser device, an optical system, and a lens.
In a laser printer of this type, the semiconductor laser device is operated based on recording data, and a laser beam containing the recording data is emitted from the semiconductor laser device.
A laser beam emitted from the semiconductor laser device is deflected and scanned by the optical system, i.e., polygon mirror or the like, and then converged by the lens on the image-forming surface of the photosensitive drum.
At this point, the surface of the photosensitive drum is already uniformly charged by the charging device.
The surface is then exposed by the laser beam, and the recording data is recorded on the surface as a static latent image.
After that, the static latent image is developed by toner in the developing device, so as to be converted into a toner image.
The toner image is transferred in the transferring device on a recording sheet supplied from the feeding device.
The toner image transferred on the recording sheet is fixed thereon by the fixing device, thus completing a series of a printing operation.
An optical scanning unit is used in the exposure device of the laser printer. The optical scanning unit includes an optical system such as a polygon mirror, and as the polygon mirror is rotated, a laser beam emitted from the semiconductor laser device is deflected and scanned.
An example of the optical scanning unit is a pre-object type optical scanning unit in which an f.theta. lens is used.
FIG. 18 shows a structure of a pre-object type optical scanning unit.
As can be seen in FIG. 18, a laser beam emitted from a semiconductor laser device 1 is converted into a parallel beam by means of a collimator lens 2.
The parallel beam reflects on a rotating reflection mirror of a polygon mirror 3, which is rotated by a motor, so as to be converted into a scanning beam.
The scanning beam, after passing through a f.theta. lens 4, reflects on a reflection mirror 5, and further passes through a cylindrical lens 6 to be converged on the surface of a photosensitive drum 7.
An example of an optical scanning unit of the type in which no polygon is used is disclosed in U.S. Pat. No. 4,943,128 (by Takada).
FIGS. 20 and 21 shows a structure of a deflector, and as is shown, a unigon mirror 12 is mounted on the rotational axis of a motor 11.
The unigon mirror 12 is surrounded by a cylindrical slit member 13, in which a slit 14 is provided on the bottom portion thereof, and a slit 15 is provided at the position opposite to the reflection surface of the unigon mirror 12 of the periphery surface.
FIG. 19 shows a structure of an optical scanning device, and as is shown, a laser beam output from a laser unit 16 is made incident on the slit 14, and reflects on the reflection surface of the unigon mirror 12. The reflection beam from the unigon mirror 12 is output through the slit 15 as a scanning beam.
The scanning beam emitted from the slit 15 passes through the f.theta. lens 17, and reflects on a reflection mirror 18. Further, the beam is irradiated on a photosensitive drum 20 through a toroidal lens 19.
After passing through the f.theta. lens 17, a portion of the scanning beam is made incident on a photosensor 22 via an optical fiber 21, and thus a recording start position on the photosensitive drum 20 is determined.
It should be noted that a polygon mirror manufactured by a method in which an aluminum reflection film is provided on a glass polished material is very expensive.
Further, the production cost of a polygon mirror can be reduced to some extent by the method in which an aluminum material is machined by a diamond cutting bite; however a cutting machine must be prepared, and the required accuracy is extremely high. Consequently, a sufficient reduction in cost cannot be expected.
Moreover, there are a plurality of reflection surfaces in a polygon mirror, creating the problem of irregular deflecting surfaces due to a machining error of each deflection surface.
In order to correct the arrangement gap, another optical system for correction must be provided, and in an optical scanning unit shown in FIG. 18, an f.theta. lens 4 or a cylindrical lens 6 is used for correction. Further, the reflection mirror 5 and the cylindrical lens 6 are arranged close to the photosensitive drum 7, complicating the structure and increasing the size.
With regard to an optical scanning unit in which no polygon mirror is used, such as shown in FIGS. 19-21, there is only one reflection surface, and therefore the device is free from the drawbacks, i.e. the requirement for machining accuracy and the displacement. However, an f.theta. lens 17 and a toroidal lens 19 must be provided.
Further, since the reflection mirror 18 and the toroidal lens 19 are arranged close to the photosensitive drum 20, complicating the structure and increasing the size, as in the case shown in FIG. 18.